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Oxygen Use in Critical Illness

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Oxygen Use in Critical Illness Oxygen Use in Critical Illness B Ronan O’Driscoll and Rachel Smith Introduction The Discovery of Oxygen History of Medical Use of Oxygen Reasons to Use Oxygen in Critical Care History of Oxygen Use in Critical Care Evidence of Harm From Excessive Use of Oxygen in Critical Care Survivors of Cardiopulmonary Resuscitation Ventilated General Critical Care Patients Stroke Patients on Critical Care Units Traumatic Brain Injury Patients on Critical Care Units Myocardial Infarction Pilot Studies of Conservative Oxygen Management for Ventilated Critical Care Patients Randomized Trials of Oxygen Therapy in Critical Care Settings Why Have Clinicians Used Too Much Oxygen for Many Decades? Current Best Practice for Oxygen Use in Critical Illness Overview Immediate Management of Critical Illness in Pre-Hospital Settings Immediate Management of Critical Illness in Hospital Settings Oxygen Use in Critical Care Units Choice of Device The Role of the Respiratory Therapist Special Uses of Oxygen Oxygen as a Driving Gas for Nebulizers Carbon Monoxide Poisoning Hyperbaric Oxygen Medically Managed Pneumothorax Oxygen and Avoidance of Perioperative Wound Infection Future Directions Oxygen is the most commonly used drug in critical care. However, because it is a gas, most clinicians and most patients do not regard it as a drug. For this reason, the use of medical oxygen over the past century has been driven by custom, practice, and “precautionary prin- ciples” rather than by scientific principles. Oxygen is a life-saving drug for patients with severe hypoxemia, but, as with all other drugs, too much can be harmful. It has been known for many decades that the administration of supplemental oxygen is hazardous for some patients with COPD and other patients who are vulnerable to retention of carbon dioxide (ie, hypercapnia). It has been recognized more recently that excessive oxygen therapy is associated with signifi- cantly increased mortality in critically ill patients, even in the absence of risk factors for hypercapnia. This paper provides a critical overview of past and present oxygen use for critically ill patients and will provide guidance for safer oxygen use in the future. [Respir Care 2019;64(10):1293–1307. © 2019 Daedalus Enterprises] RESPIRATORY CARE • OCTOBER 2019 VOL 64 NO 10 1293 OXYGEN USE IN CRITICAL ILLNESS Introduction oxygen was much overused.3 He exposed 3 fallacies that will be discussed further in this review. First, there was a Critical illnesses such as major trauma, pneumonia, sep- belief that exposure to supplemental oxygen at concentra- sis, or heart failure can cause dangerously low blood ox- tions up to 60% was without adverse effects. Second, it ygen levels (ie, hypoxemia), which can cause tissue dam- was thought that patients at risk of developing arterial age and death.1 For this reason, avoidance of hypoxemia is hypoxemia could be protected by administering supple- a central tenet of the emergency medical response to crit- mental oxygen. Third, it was thought that routine admin- ical illness. The first two elements of the “ABC” of emer- istration of supplemental oxygen was useful, harmless, gency care refer to ensuring oxygenation by attention to and clinically indicated on a prophylactic basis. the airway and breathing, and the third element (ie, circu- Since 2002, there have been numerous publications that lation) is also critical to deliver oxygen to the tissues. For have supported Downs’ view that supplemental oxygen these reasons, oxygen therapy became a central compo- has been overused, but clinical practice has been slow to nent of resuscitation and critical illness management dur- evolve. This review will explain the principles underlying ing the 20th century. The prevailing philosophy regarding contemporary oxygen therapy together with an outline of oxygen use during that century was that “more is better.” oxygen use in the management of critically ill patients in There was no awareness in the 20th century that high- the past and guidance for safe oxygen use in the manage- concentration oxygen therapy could be harmful apart from ment of critical illness in the future. The authors will try to special circumstances such as in the care of hypercapnic explain why clinical practice in this area has been so slow patients with exacerbations of COPD, who were known to change and why it has been so difficult to translate since the 1960s to require controlled oxygen therapy, and research findings into routine care. in the care of premature babies, in whom excessive oxygen therapy was recognized since the 1950s as a cause of The Discovery of Oxygen blindness.1,2 There was no reliable way of knowing if a patient was adequately oxygenated until they arrived at the It was thought in the 18th century that the process of hospital and had invasive blood gas measurements. Out- combustion gave off a chemical called phlogiston; it is side of critical care units, blood gas tests could be under- now known that combustion (and animal respiration) ac- taken only intermittently, and repeated arterial sampling is tually involves the consumption of oxygen. An English traumatic for patients. In these circumstances, it is under- scientist, theologian, and polymath named Joseph Priest- standable that 20th-century clinicians managing patients ley (the inventor of soda water and thus of all fizzy drinks) with critical illness administered oxygen lavishly to avoid isolated a colorless gas by heating mercuric oxide. This hypoxemia, and they had no concerns regarding possible gas allowed candles to burn brighter and for longer, and it harm from hyperoxemia outside of the special groups of prolonged the life of mice in sealed containers. Priestley patients mentioned above. thought he had discovered something he called dephlogis- However, with the advent of ubiquitous cheap and re- ticated air. A similar discovery was made by a Swedish liable pulse oximeters in the early 21st century, alongside chemist named Carl Scheele at about the same time, but increasing knowledge regarding the dangers of hyperox- Priestley got most of the credit because he published first. emia and the lack of evidence of any benefit from aggres- It was the French chemist, Lavoisier, who recognized that sive oxygen therapy, the use of oxygen in critical care in Priestley and Scheele had actually discovered a new gas, the 21st century should be very different from what hap- which he called oxygen. It is fascinating to observe that pened in the previous century. In 2002, in his prestigious Priestley recognized the potential medical uses of oxygen, Donald F Egan Scientific lecture to the 48th International as well as the potential risks of accelerating oxidative pro- Respiratory Congress of the American Association for Re- cesses when he watched candles flare up in this new gas. spiratory Care, John Downs concluded that supplemental He issued the following prophetic words in 1776: From the greater strength and vivacity of the flame of Dr O’Driscoll and Ms Smith are affiliated with the Department of Re- a candle, in this pure air, it may be conjectured, that it spiratory Medicine, Salford Royal University Hospital, Salford, United might be particularly salutary to the lungs in certain Kingdom. morbid cases, when the common air would not be sufficient to carry off the phlogistic putrid effluvium The authors have disclosed no conflicts of interest. fast enough. But, perhaps, we may also infer from Correspondence: B Ronan O’Driscoll MD FRCP, Department of Respi- these experiments, that though pure dephlogisticated ratory Medicine, Salford Royal University Hospital, Stott Lane, Salford air might be useful as a medicine, it might not be so M6 8HD, United Kingdom. E-mail: [email protected]. proper for us in the usually healthy state of the body; for as a candle burns out much faster in dephlogisiti- DOI: 10.4187/respcare.07044 cated than in common air, so we might, as may be 1294 RESPIRATORY CARE • OCTOBER 2019 VOL 64 NO 10 OXYGEN USE IN CRITICAL ILLNESS said, live out too fast, and the animal powers be too the main fuel for the metabolism of the cells and organs of soon exhausted in this pure kind of air. animals and humans, a major fall in the tissue oxygen level will produce rapid and severe injury to several organs, History of Medical Use of Oxygen especially the brain, which is the most vulnerable organ in hypoxic conditions. Hypoxia is common in critical illness, Attempts to use oxygen for medical benefit began in the and hypoxic brain damage is one of the most feared com- late 18th century within a few years of the discovery of plications of hypoxia. Furthermore, a high proportion of oxygen, and these attempts continued through the 19th patients in critical care units require mechanical ventila- century. As early as 1798, Thomas Beddoes founded the tion, which usually involves the use of supplemental ox- pneumatic institution for inhalation gas therapy in Bristol ygen above the level of room air (ie, 21% oxygen). It is for using oxygen and other gases, including nitrous oxide (ie, these reasons that supplemental oxygen is administered to laughing gas) manufactured by the famous scientist, Hum- the majority of patients in critical care units. phrey Davy. For more than 100 years, the most common Patients with medical emergencies who are hypoxemic indication for oxygen use was in the treatment of pulmo- are more likely to die than those with normal blood oxy- nary tuberculosis, which was a common cause of death in gen levels.5 However; it is difficult to know in most cases the 19th century. Unfortunately, supplies of oxygen were whether the increased risk of death is a consequence of the limited at that time, and the intermittent application of low blood oxygen level itself or just a marker of disease oxygen via primitive face masks was probably of little severity.
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